Asteroid Belts, Possible Planets Around Epsilon Eridani

by Paul Gilster on October 28, 2008

Two asteroid belts around Epsilon Eridani? So we learned yesterday, a fascinating find and one I want to discuss today, but only after celebrating Epsilon Eridani itself. Can any star have a more interesting pedigree? This is one of the Project Ozma stars, the other being Tau Ceti, that Frank Drake targeted in the first attempt to listen in on extraterrestrial civilizations. The Centauri stars seemed less likely then, in an era when multiple systems were thought to be hostile to planetary formation. But Epsilon Eridani and Tau Ceti were both single, Sun-like stars, surely possible homes to planets not much different from ours.

Or so we thought. We’ve since learned that Tau Ceti’s chances as a home to flourishing civilizations are diminished by the likelihood of intense cometary bombardment, while Epsilon Eridani itself is young enough (850 million years) that any parallel with our own Solar System, where life has had billions of years to attain technology, breaks down. But these stars are close enough to us to make them realistic targets for interstellar probes, assuming we develop the needed technologies and have the will to launch them. And if exploration in our own system has taught us anything, it’s that we should be prepared to be surprised, no matter how well established are our preconceptions.

So we move 10.5 light years in the direction of the constellation Eridanus to a star that looks a lot like ours once did, a point that Massimo Marengo (Harvard-Smithsonian Center for Astrophysics) is quick to note. “Studying Epsilon Eridani is like having a time machine to look at our solar system when it was young,” says Marengo, co-author of the discovery paper. That calls up the possibility that this system may evolve as ours has, perhaps one day yielding civilizations of its own, even if none were available for Frank Drake to overhear in 1960.

The new findings show that Epsilon Eridani has an asteroid belt at a distance of some 3 AU, more or less in the same place as the asteroid belt around our Sun. A second belt, found using the Spitzer Space Telescope, exists at about 20 AU, in the area where Uranus orbits in our system. Still further out, extending some 35 to 100 AU from the central star, is a ring of icy materials reminiscent of our own Kuiper Belt, but holding 100 times more material. Current thinking is that when the Sun was Epsilon Eridani’s age, the Kuiper Belt looked more or less the same, with much of its material swept away in the Late Heavy Bombardment.

Image: This artist’s conception shows the closest known planetary system to our own, Epsilon Eridani. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. The system’s inner asteroid belt appears as the yellowish ring around the star, while the outer asteroid belt is in the foreground. The outermost comet ring is too far out to be seen, but comets originating from it are shown in the upper right corner. Credit: NASA/JPL-Caltech.

The intriguing gaps between the rings of material around this star are laden with possibility. We already have some radial velocity evidence for the existence of a Jupiter-mass planet in an elliptical orbit with a semi-major axis of 3.4 AU and an orbital period of 6.9 years. That would place this world near the location of the innermost belt identified in this work. But it would also create a problem, according to the paper: Ranging from 1 AU to 5.8 AU as it moves through its orbit, such a planet “…would quickly clear that region not only of dust particles but also the parent planetesimal belt needed to resupply them, inconsistent with the dust distribution implied by the Spitzer observations.” The problem can be eased if the degree of this planet’s orbital eccentricity is diminished, which would allow an orbit entirely within the innermost belt.

The paper goes on to offer the evidence for three possible planets in this system:

Planet A: the long-suspected but still unconfirmed Jovian-mass planet with aorb = 3.4 AU that may be associated with the innermost warm debris belt detected by Spitzer. Planet B: also perhaps Jovian-mass, associated with the second warm debris belt at r ∼ 20 AU inferred in this paper, also possibly helping define the sub-mm ring inner edge via 2:1 resonance, keeping the zone between 20 and 35 AU clear… Planet C: located at r ∼ 35 AU, with mass less than a few × 0.1 MJup according to dynamical models, preventing small grains from drifting inward past the inner edge of the sub-mm ring.

It’s intriguing to note that while Spitzer would not have been able to detect it, at least one of these worlds may be within range of the James Webb Space Telescope, scheduled for launch in the next decade. The image below shows a comparison between our Solar System and what we’ve thus far discovered around Epsilon Eridani. Note the size of that outer belt of icy materials compared to the Kuiper Belt!

Image: This artist’s diagram compares the Epsilon Eridani system to our own solar system. The two systems are structured similarly, and both host asteroids (brown), comets (blue) and planets (white dots). Credit: NASA/JPL-Caltech.

Both of the inner planetary possibilities orbit near the rim of one of the asteroid belts, and the third would be located near the rim of the outer ring of comets. Future work should tell us much more, including the ever lively question of whether terrestrial worlds may lurk inside the innermost asteroid belt. Whatever the case, a star close enough to be an early target for any interstellar technology we develop will always hold our interest. The paper is Backman et al., “Epsilon Eridani’s Planetary Debris Disk: Structure and Dynamics based on Spitzer and CSO Observations,” slated for publication in The Astrophysical Journal and available online.

Good news! In a couple billion years, when our current neighborhood is getting rundown and unliveable, there should be a new, up-and-coming neighborhood that we can all move to!

Seriously, though, I’ve always liked Epsilon Eridani as a candidate star for eventual exploration and possible colonization. The results of this study seem to lend some support to that goal and suggest that more observations are needed.

Well this is interesting… constraining the ~3.4 AU jovian to a low-eccentricity orbit makes the HZ of Epsilon Eridani look a lot safer for terrestrial planets. Not entirely sure why the authors decided to rename the planets: the equivalences are Epsilon Eridani b=Planet A, Epsilon Eridani c=Planet C, while Planet B is newly proposed. This is somewhat confusing.

As for suitability of colonisation, living planetside looks like it might have a rather high risk of impacts.

For those not familiar with Alastair Reynolds’ work, this snip from the Wikipedia explains the Melding Plague:

“The Melding Plague is a nanotech virus that attacks anything that has nanotechnology present within it and does not discriminate between human and machine. It attempts to meld the nanomachines and implants that are commonly present in the bodies of humans, with the structure of their body on a cellular level. This results in horrific, uncontrollable modifications to the body of whoever is infected and almost inevitably leads to death.”

The presence of the inner asteriod belt dust ring at 3 au and the orbital ecentricity of Eps. Eridani is a major conflict. Either the eccentricity suggested by its doppler measures is false (possibly it contains the siganal of another planet), in which case all highly eccentric orbits must be suspect; or we are seeing the Eps. Eridani system at a moment of dynamic instability. It may have ejected a planet within the last few million years.

Good idea! That’s a reasonable hypothesis or two. Quite a few singular exoplanets might be hiding another with their radial velocity signal’s supposed eccentricity. I think there’s a paper on the arXiv along those lines too.

Actually the RV data is somewhat suspicious: the systemic console has several different combinations of datasets for this system, and each one can get you very different parameters for the planet. I’m also not too convinced by the astrometric results given in Benedict et al. (2006) as the paper in question appears to contradict itself (the orbital elements given don’t seem to give the orientation depicted in the diagram or the periastron position angle mentioned in the text), but there might be some weird conventions for astrometric orbital elements that I haven’t taken into account.

Every day we are getting closer and closer to finding a planet which we can claim as a good target for an interstellar mission. This graph indicates that the year will be 2011 when we discover an Earth-mass exoplanet.

If such a planet were discovered there would be a golden opportunity of public excitement which we could use to create broad interest in an interstellar mission.

But my question is this? If it were discovered tomorrow or a year from now would we be able to make a compelling case that resources need to be spent on an interstellar mission? Which mission? We have no consensus as to what mission is at the top of the list. Nor do we have any mechanism to create such a list. I fear that, at that point in time, we might fumble the ball and the attention and funding would inevitably turn back to sol system development and discovery.

Abstract: 60% of the A star members of the 12 Myr old beta Pictoris moving group (BPMG) show significant excess emission in the mid-infrared, several million years after the proto-planetary disk is thought to disperse. Theoretical models suggest this peak may coincide with the formation of Pluto-sized planetesimals in the disk, stirring smaller bodies into collisional destruction.

Here we present resolved mid-infrared imaging of the disk of eta Tel (A0V in the BPMG) and consider its implications for the state of planet formation in this system. eta Tel was observed at 11.7 and 18.3um using T-ReCS on Gemini South. The resulting images were compared to simple disk models to constrain the radial distribution of the emitting material. The emission observed at 18.3um is shown to be significantly extended beyond the PSF along a position angle 8 degrees. This is the first time dust emission has been resolved around eta Tel.

Modelling indicates that the extension arises from an edge-on disk of radius 0.5 arcsec (~24 AU). Combining the spatial constraints from the imaging with those from the spectral energy distribution shows that >50% of the 18um emission comes from an unresolved dust component at ~4 AU.

The radial structure of the eta Tel debris disk is reminiscent of the Solar System, suggesting that this is a young Solar System analogue. For an age of 12Myr, both the radius and dust level of the extended cooler component are consistent with self-stirring models for a protoplanetary disk of 0.7 times minimum mass solar nebula. The origin of the hot dust component may arise in an asteroid belt undergoing collisional destruction, or in massive collisions in ongoing terrestrial planet formation.

Quote from Gerald Nordley
“Debra Fisher (of the California and Carnegie planet search team) has been
pointing out for years that eccentricities should be regarded as rough upper
limits, as the presence of multiple planets can (not always) produce an RV curve
that looks like one highly eccentric planet.”

If you asume the doppler signal is two planets and take the infered planets from the limits of the system’s Kuipier belt and outer asteroid belt, the you have a system with a asteroid belt at 3 au and at least 4 gas giants, which looks similar enough to our own system that terrestrial planets in the inner part of the system look like a good possibility. With improved inferonomic nulling or masking of the star, the zodiacal dust from the inner asteroid belt should show up and we could get some idea of the size of them from the bands they carve out of the dust cloud.

To Darnell, I would be a lot more confident about planets around Alpha Centauri if we could pick up the infared excess from an asteroid belt around one or both stars at 3-4 au, which is where the dynamics of planetary formation indicate there should be one.

Abstract: Eps Ind A is one of the nearest sun-like stars, located only 3.6 pc away. It is known to host a binary brown dwarf companion, Eps Ind Ba/Bb, at a large projected separation of 6.7″, but radial velocity measurements imply that an additional, yet unseen component is present in the system, much closer to Eps Ind A.

Previous direct imaging has excluded the presence of any stellar or high-mass brown dwarf companion at small separations, indicating that the unseen companion may be a low-mass brown dwarf or high-mass planet.

We present the results of a deep high-contrast imaging search for the companion, using active angular differential imaging (aADI) at 4 micron, a particularly powerful technique for planet searches around nearby and relatively old stars. We also develop an additional PSF reference subtraction scheme based on locally optimized combination of images (LOCI) to further enhance the detection limits.

No companion is seen in the images, although we are sensitive to significantly lower masses than previously achieved. Combining the imaging data with the known radial velocity trend, we constrain the properties of the companion to within approximately 5-20 Mjup, 10-20 AU, and i > 20 deg, unless it is an exotic stellar remnant. The results also imply that the system is probably older than the frequently assumed age of ~1 Gyr.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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